In power devices, a bushing is used for connection to an external power source (power line). The bushing includes an outer-cover insulator having a shed so as to secure a long insulating distance, and also includes a capacitor which surrounds a central conductor and in which several conductive layers are wound around an insulating paper to reduce an electric field. An epoxy resin fills the space between the insulating paper and the conductive layers so as to surround the insulating paper and the conductive layers.
Since the insulating paper is used, the epoxy resin may not fill the inside of the bushing sufficiently to reach the position of the central conductor, and since the extent of expansion and shrinkage of epoxy is different from the extent of expansion and shrinkage of the insulating paper, there is a problem in that many gaps are generated between the epoxy resin and the insulating paper during the manufacture of a capacitor bushing.
An aluminum thin film is generally used as the conductive layer. The aluminum thin film is different from the epoxy resin in the extent of expansion and shrinkage thereof, like the insulating paper, and the bonding force between the aluminum thin film and the epoxy resin is weak. Therefore, there is a problem in that moldability is poor due to shrinkage and expansion during the thermal expansion and curing of the epoxy resin.
The aluminum thin film is an element that interferes with filling of epoxy, and has a problem in that an electric field is concentrated on a sharp portion formed during cutting.
In addition, synthetic fibers are used to solve the problem in which the insulating paper disturbs sufficient filling of the epoxy resin in the bushing, and the extent of expansion and shrinkage of the synthetic fibers is different from that of the epoxy resin. In particular, the epoxy resin expands and shrinks most in the longitudinal direction of the bushing. Since wefts or warps constituting the synthetic fibers extend in the longitudinal direction of the bushing, problems according to expansion and shrinkage occur between the epoxy resin and the synthetic fibers.
In addition, the epoxy resin expands and shrinks due to heat. If bubbles are present in the epoxy resin while expanding during filling of the epoxy resin, the bubbles grow to prevent complete filling of a filling layer in the bushing.